The invention is directed to the recovery of metal ions by electrodeposition thereof from waste water streams such as those obtained in the metal finishing industry. By "metal" in this application, we refer particularly to electrodepositable ("platable") metals such as copper (Cu), zinc (Zn), cadmium (Cd), nickel (Ni) and precious metals such as silver (Ag) and gold (Au). The waste water streams typically contain one or more metals in a concentration less than about 1000 parts per million (ppm), often less than about 500 ppm. The lower the concentration of the metal in the stream the more difficult it is to recover the metal economically. Streams with concentrations less than about 1000 ppm have relatively few ions per unit volume and are generally referred to as `dilute aqueous metal-bearing streams`; they will be referred to herein as "problem streams" because of the well-recognized problem of recovering their metal values economically.
This invention is especially directed to scavenging problem streams and reducing their concentrations to as low as about 0.5 ppm to about 10 ppm ("low end concentration") depending upon the flow volume, the residence time in the cell, and stream characteristics. Streams with greater concentrations in the range from about 1 gram per liter (g/l) to about 10 g/l are far more easily scavenged than the problem streams, and of course, our cathodes may be profitably used with such relatively metal-rich streams.
Statistically, if most of the relatively few ions (problem streams being dilute) are to find their way to a cathode surface, the cathode must be a high surface area electrode. By "high surface area" electrode we refer to a three-dimensional electrode in which the ratio of the active surface of the electrode to the geometric volume of the electrode is at least 150 and preferably from about 500 to about 2000. By "geometric volume" we refer to the product of the length l, the width w, and the thickness or height h, of a parallelepiped electrode. Thus its volume is lwh, and the geometric surface area of one face is lw. Such high surface area is provided by reticulated electrodes ("reticulates") such as are disclosed in copending commonly assigned U.S. patent application Ser. No. 286,551 (hereafter "the '551 application"), filed July 24, 1981, issued on Aug. 16, 1983, as U.S. Pat. No. 4,399,020 the disclosure of which is incorporated by reference thereto as if fully set forth herein.
Our invention is specifically concerned with directly metal-electroplated electrodes referred to herein as "single deposit electrodes" (for brevity, "SDE") because the electrode is plated with a single plating process, namely electroplating. These electrodes are made from an electrically conductive open cell foam ("ECOF" for brevity) referred to as "large pore" foam having a void fraction in the range from about 0.5 to about 0.98 and more preferably from about 0.75-0.98.
"Void fraction" and "porosity" are synonymous terms except that the former is expressed as an arithmetic fraction, and the latter is expressed as a percentage. The pores of an ECOF are easily visible, being in the range from about 0.15 mm to about 2 mm in equivalent diameter ("pore diameter"). Such pores are outlined in the foam by a mass of filaments (thin strands or ribbons) so as to produce a typical three-dimensional "filiform open cell foam morphology" referred to herein as a "foam reticulate", which is different from and clearly distinguishable over a woven fabric.
Essentially all the filaments of an ECOF are coated substantially uniformly with carbon particles which adhere to the filaments, and the pores are in open fluid communication with each other, and so remain after the ECOF is metal-plated, so as to afford through-passage for a metal-bearing aqueous solution ("solution porous") from one exterior surface of the reticulate to an opposite exterior surface. By "coated substantially uniformly" we mean that each unit of active area is coated with substantially the same weight of material (in this case, carbon). The ECOF is essentially identical with, and has essentially the same pore size, porosity or void fraction, and pressure drop characteristics as the uncoated electrically non-conductive foam ("ENCF") from which the ECOF was derived.
The desirability of "flow-through" electrodes is well-recognized (see for example the beds of conductive particles used in U.S. Pat. Nos. 3,696,201; 3,954,594; 3,977,951; 4,292,160; and, 4,313,813 inter alia), and numerous efforts have been made to provide such electrodes without adversely affecting the operating efficiency of the cell. Particle beds were used to provide the requisite area. A more recent development is of record in the aforementioned '551 application.
In our invention, as in the cell box described in the '551 application, plural alternating cathodes and anodes are connected to bus bars located on a non-conductive cell box, so that in operation, the cell scavenges metal contaminants by deposition thereof on the cathodes. A preferred anode is a dimensionally stable anode ("DSA.RTM.") such as is described in U.S. Pat. Nos. 3,632,498; 3,711,385; 3,751,296, inter alia, the disclosures of which are incorporated by reference thereto as if fully set forth herein. Our cathode may be substituted for the cathode used in the prior art cell box.
The preferred cathode in the '551 application comprises an organic polymer (for example, polyurethane) open cell foam substrate having mostly interconnecting pores upon which substrate there is deposited electrolessly, then electrolytically, (hence referred to as a "dual deposit electrode" or "DDE" for brevity), a metal or alloy thereof. The drawback of this DDE is that it requires plural processing steps which are time-consuming, requires the use of much equipment, and uses too much raw material. The DDE is costly to fabricate.
Apart from and unrelated to the cost of a DDE, in other liquid permeable prior art cells, the use of a separator means ("separator") in the path of flow of the solution being scavenged, appeared to be an unavoidable necessity, particularly if particles are to be confined. By a "separator" we refer to a porous insulating sheet of plastic fiber cloth, polypropylene, and similar "small pore" materials having pores smaller than about 0.1 mm pore diameter, and include `membranes`.
Many efforts have been made in the prior art to obviate the use of a separator of any kind because of its inherent relatively high resistance to flow which was exacerbated since it was so easily clogged in a scavenging service. For example, as early as in U.S. Pat. No. 2,616,165 an open mesh woven material, or a thin porous mat of fibers, was shown as having been coated with metal to make the mat conductive. The impracticality of coating a foam reticulate by such methods was overcome many years later with the advent of the DDE referred to hereinabove. U.S. Pat. No. 3,969,201 taught the use of a bed of graphite particles which bed was liquid permeable, but required confinement with a membrane; the concept of providing a non-conductive foam coated with carbon particles was never suggested.
A method for preparing a DDE is disclosed in "Characterization of Reticulate, Three-Dimensional Electrodes" by Tentorio, A. et al, Jour. of Appl. Electrochem., 8 at 195-205, 1978. Though the SDE and DDE have comparable performance in service, the cost of fabricating a DDE is significantly higher than that of a SDE.
A desire to produce an SDE raises the problem of providing adequate conductivity for a foam reticulate and affects the choice and design of the current conductor for producing it. This is a serious problem even when the electrode is a battery plate having a skeleton of welded metallic fibers and one side of the plate is compressed in the form of a base on which strips of nickel are welded, as disclosed in U.S. Pat. No. 3,600,227.
Reverting therefore to the DDE, it was evident that a desirable goal was to deposit metal on the thin filaments or strands in a single operation, in enough concentration to give the requisite conductivity at minimum expense. This was the formula for an effective SDE. The problem was to reduce the formula to practice.
Initial attempts to fabricate an SDE failed partly because it was not known that if an ECOF could be obtained having a resistivity in the "right" range, the ECOF could be electrodeposited with a thin substantially uniform deposit of a platable metal if the ECOF was suitably supported over one face with a current conductor. Most commercially available ECOFs, whether organic carbon-coated ECOFs such as polyurethane, or a carbon-coated ENCF such as ceramic or glass foam, had resistivities in the range above about 3000 ohm-cm and could not be plated. Even a resistivity in the range from about 1000 ohm-cm to about 3000 ohm-cm was found generally to be too high to produce a thin substantially uniform deposit without (i) damaging the ECOF at the current density required to deposit metal, or, (ii) over-polarizing the cell, as is evidenced by an excessive energy consumption in the cell.
To counter this problem the prior art resorted to numerous structural configurations for their electrodes, as for example, the toothed electrodes bonded to the organic foam with a metal deposit, disclosed in U.S. Pat. No. 4,336,124.
The problem of low conductivity was found to be overcome when the ECOF was wrapped around a copper tube, except that it caused the ECOF to be weakly bonded to the surface of the tube. This was fortuitous from the point of view of improving heat transfer after pyrolysis of the ECOF, as disclosed in U.S. Pat. No. 4,136,428. It was of little use relative to electrowinning metals from problem streams as the electroplated foam on a tube surface was not liquid-permeable in a direction at right angle to its circumferential surface (either before or after pyrolysis of the foam). Moreover, electrodepositing copper from an air agitated standard copper sulfate electroplating solution was far-removed from scavenging metals from a problem stream. Nevertheless, the possibility that an ECOF containing carbon which was coated on the filaments of the foam (also referred to as graphite-coated ECOF) may be made sufficiently conductive within a preselected range, to allow directly electroplating the ECOF, in a single step, from a dilute metal-bearing solution, sparked the improvement of this invention over the reticulated cathode of the '551 application.
Reticulates having a void fraction in the range from about 0.5-0.8 (porosity of 50-80%) are sometimes referred to as "felt-like porous bodies"; and, those having a void fraction in the upper range from about 0.75-0.98 are referred to as "sponge metals". See "Characteristics and Applications of Sponge Metal" by Eiji Kamijo and Masaaki Honda, in Chemical Economy and Engineering Review, published by Chemical Economy Research Institute (Japan), December 1975, the disclosure of which is incorporated by reference thereto as if fully set forth herein. Though there is no enabling disclosure as to how such sponge metals may be prepared, their pores are said to have a spherical shape, and their sintered metal framework extends in all directions in a continuous reticulated structure, having high surface area well-suited for use as electrodes. Except that such sintered metal electrodes are not disposable or economical in scavenging problem streams.
The matter of cost of a cathode to be used in this "problem stream service" is critical since `common` metals such as copper and zinc are not routinely recovered for reuse as the cost of doing so is economically unjustifiable. Such metals are electrowon for environmental considerations, not for their intrinsic value. Therefore, the `loaded` or `filled` cathodes are simply discarded as scrap. Hence this invention is more particularly directed to the production of disposable cathode reticulates having an ECOF substrate, and specifically to cathodes derived from and retaining a skeletal framework ("skeleton") of a synthetic resinous ECOF ("SRECOF") such as polyurethane. When the price of common metals justifies it, the filled cathodes may be used as anodes and the metals recovered; or the metals may be recovered by any other known process.
In light of the foregoing it will now be apparent that it has long been theoretically established that a sufficiently large number of essentially pure Cu, Ni, Cd, Zn, Ag or Au reticulates with sufficient area, alternated with anodes, will effectively scavenge a problem stream containing ions of one or more of the metals; but the cost of such pure metal cathodes relegates their use to academic studies.